Planetary gearbox for gas turbine engine

ABSTRACT

In one aspect, there is provided a planetary gearbox, comprising a sun gear, a plurality of planet gear assemblies, each planet gear assembly having a main gear meshed with the sun gear, a fore lateral gear and an aft lateral gear disposed on opposite sides of the main gear and rotating therewith, a diameter of the main gear being different than a diameter of the fore and aft lateral gears, a planet carrier rotatably supporting at least some of the planet gear assemblies, and at least one fore ring gear meshed with the fore lateral gears, at least one aft ring gear meshed with the aft lateral gears, wherein one of the sun gear, the planet carrier, and the ring gears is configured to be operatively connected to an input, one is configured to be operatively connected to an output, and rotation of a remaining one is limited.

TECHNICAL FIELD

The application generally relates to aircraft engines and, moreparticularly, to gearboxes used in an aircraft engine such as a gasturbine engine.

BACKGROUND OF THE ART

Turboprops are gas turbine engines coupled to a propeller via areduction gearbox. Contrary to a turbofan engine, in which energy fromthe jet is used to generate thrust, a turboprop turbine converts thisenergy in mechanical energy. The turbine is then used to drive thepropeller. However, the rotational speed of the turbine is too high tobe directly coupled to the propeller. Accordingly, a reduction gearboxis used to reduce the rotational speed of the propeller relative to theturbine and to increase the torque generated by the turbine. Gearboxesadd weight and complexity to the engine, and room for improvementexists.

SUMMARY

In one aspect, there is provided a planetary gearbox, comprising a sungear, a plurality of planet gear assemblies, each planet gear assemblyhaving a main gear meshed with the sun gear, a fore lateral gear and anaft lateral gear disposed on opposite sides of the main gear androtating therewith, a diameter of the main gear being different than adiameter of the fore and aft lateral gears, a planet carrier rotatablysupporting at least some of the planet gear assemblies, and at least onefore ring gear meshed with the fore lateral gears, at least one aft ringgear meshed with the aft lateral gears, wherein one of the sun gear, theplanet carrier, and the ring gears is configured to be operativelyconnected to an input, one is configured to be operatively connected toan output, and rotation of a remaining one is limited.

In another aspect, there is provided a gas turbine engine comprising acompressor, a combustor and a turbine, the turbine having a firstsection for driving the compressor and a second section driving a load,the second section of the turbine operatively connected to a sun gear ofa planetary gearbox, the sun gear meshed with main gears of a pluralityof planet gear assemblies pivotally mounted on a planet carrier, theplanet gear assemblies each having a fore lateral gear and an aftlateral gear of a diameter different than a main gear diameter, the foreand aft lateral gears disposed on opposite sides of the main gears, atleast one fore ring gear meshed with the fore lateral gears, at leastone aft ring gear meshed with the aft lateral gears, one of the ringgears and the planet carrier driving the load by rotation, whilerotation of another one of the ring gears and the planet carrier islimited.

In yet another aspect, there is provided a method for changing arotational speed of a first rotating component relative to a secondrotating component, comprising receiving a torque from the firstrotating component using a sun gear; transmitting at a first rotationalspeed ratio a rotation of the sun gear to a plurality of main gears of aplurality of planet gear assemblies rotatably mounted onto a planetcarrier, transmitting at second rotational speed ratio a rotation of themain gears to one of the planet carrier and ring gears while limitingrotation of another one of the planet carrier and the ring gears, thering gears meshed to gears disposed on opposite sides of and rotatingwith the main gears, and dividing between the ring gears a loadtransmitted from the second rotating component.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 is a cross-sectional view of a portion of the gas turbine engineillustrating a planetary gearbox;

FIG. 3 is a tri-dimensional view of the planetary gearbox of FIG. 2; and

FIG. 4 is a cross-sectional view along line 4-4 of the planetary gearboxof FIG. 3.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight and configured for driving a load 12, suchas, but not limited to, a propeller or a helicopter rotor. Depending onthe intended use, the engine 10 may be any suitable aircraft engine, andmay be configured as a turboprop engine or a turboshaft engine. The gasturbine engine 10 generally comprises in serial flow communication acompressor section 14 for pressurizing the air, a combustor 16 in whichthe compressed air is mixed with fuel and ignited for generating anannular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases.

The exemplary embodiment shown in FIG. 1 is a “reverse-flow” enginebecause gases flow from the inlet 17, at a rear portion of the engine10, to the exhaust outlet 19, at a front portion of the engine 10. Thisis in contrast to “through-flow” gas turbine engines in which gases flowthrough the core of the engine from a front portion to a rear portion.The engine 10 may be a reverse-flow engine (as illustrated) or athrough-flow engine.

In the illustrated embodiment, the turbine section 18 has ahigh-pressure turbine 18A in driving engagement with a high-pressurecompressor 14A. The high-pressure turbine 18A and the high-pressurecompressor 14A are mounted on a high-pressure shaft 15. The turbine 18has a low-pressure turbine, also known as power turbine 18B configuredto drive the load 12. The power turbine 18B is configured to drive alow-pressure compressor 14B through a low-pressure shaft 22. A reductiongearbox 20 is configured to connect the low-pressure shaft 22 thatsupports the power turbine 18B to a shaft 24 that is in drivingengagement with the load 12, while providing a reduction speed ratiotherebetween.

The reduction gearbox 20 allows the load 12 to be driven at a givenspeed, which is different than the rotational speed of the low-pressureturbine 18B. The reduction gearbox 20 allows both the load 12 and thelow-pressure turbine 18B to rotate at their respective optimal speedwhich are different. In the embodiment shown, the reduction gearbox 20is axially mounted at the front end of the engine 10.

Now referring to FIGS. 1-4, the reduction gearbox 20 comprises aplanetary gearbox 30, also known as epicyclic gear train, epicyclicgearbox, etc, but referred to as a planetary gearbox 30 herein forclarity. The planetary gearbox 30 has a sun gear 32 mounted on a sungear connector 34 configured to be connected to a layshaft 22 a that isconnected the low-pressure shaft 22. In an alternate embodiment, the sungear 32 is mounted directly onto the layshaft 22 a that is connected tothe low-pressure shaft 22. The layshaft 22 a, also known as a torqueshaft, is configured to allow flexibility from deflection or othercontributor between the turbine section 18 and the reduction gearbox 20.In operation, the layshaft 22 a is designed to twist along itsrotational axis by a certain amount. The twist of the layshaft 22 a ismonitored to indicate the actual torque that it transmits. The planetarygearbox 30 further has a set of planet gear assemblies 36 rotatablymounted on shafts 38—three planet gear assemblies 36 are shown, althoughthe planetary gearbox 30 could have two or more planet gear assemblies36. In the embodiment shown, all shafts 38 of the set of planet gearassemblies 36 are connected to a planet carrier 40, the planet gearassemblies 36 rotating onto the shafts 38. In a particular embodiment,the planetary gearbox 30 comprises a plurality of planet gear assemblies36. At least some of the plurality of assemblies 36 are mounted on theplanet carrier 40, while others may simply rotate while not beingconnected to the planer carrier 40. In the illustrated embodiment,bearings 42 are disposed between the shafts 38 and the planet gearassemblies 36. The bearings 42 are shown as plain or oil film bearings.In an alternate embodiment, rolling element bearings may be used amongother possible arrangements. In the illustrated embodiment, the planetcarrier 40 has a connector 44 adapted to be coupled to the shaft 24 ofthe load 12. Alternatively, the planet carrier 40 may be mounteddirectly to the shaft 24. In an alternate embodiment, the planet carrier40 is a zero-twist carrier to reduce twist deflection under torque bydriving the planet gear assemblies 36 from an axial positioncorresponding to a symmetry plane of the planet gear assemblies 36. In aparticular embodiment, the zero-twist carrier is as described in U.S.Pat. No. 6,663,530 which is incorporated herein by reference in itsentirety. Alternatively, stiffness of the shaft 38 may be varied toreduce the deflection that is transmitted to the planet gear assemblies36.

Each planet gear assembly 36 has a main gear 46, a fore and aft lateralgears 48 disposed on opposite sides of the main gear 46. The fore andaft lateral gears 48 rotate integrally with the main gears 46. The maingears 46 are meshed with the sun gear 32. In the illustrated embodiment,the main gears 46 and the sun gear 32 are spur gears, but other types ofgears may be used, such as helical gears. In the embodiment shown, adiameter 50 of the sun gear 32 is inferior to a diameter 52 of the maingears 46 to create a first rotational speed ratio to the planetarygearbox 30, between the sun gear 32 and the main gears 46 of the planetgears assemblies 36.

Ring gears 54 are meshed with the fore and aft lateral gears 48 of theplanet gears assemblies 36. The ring gears 54 consist of two halves andare disposed symmetrically on each side of the main gears 46 so that thereaction load on the bearings 42 is equalised along their longitudinalaxis. The gears 48 and 54 may be spur gears (internal spur gear in thecase of the ring gear 54). In the illustrated embodiment, the lateralgears 48 and the ring gears 54 are helical gears. Helical gears may bequieter. In a particular embodiment, teeth of the fore lateral gear areangled in an opposite way relative to teeth of the aft lateral gear suchthat the fore and aft lateral gears are mirrored relative to oneanother. In operation, the main gears 46 of such a particular embodimentself-center under torque relative to the sun gear 32. This may enhancethe load sharing between the ring gears 54. In the embodiment shown, adiameter 56 of the lateral gears 48 is inferior to the diameter 52 ofthe main gears 46. Accordingly, a second rotational speed ratio betweenthe planet gear assemblies 36 and the ring gears 54, or between theplanet gears assemblies 36 and the planet carrier 40, is generated inthe planetary gearbox 30.

The planetary gearbox 30 provides a rotational speed ratio between thesun gear 32 and the planet carrier 40 that would require at least twoconventional planetary gearboxes to achieve. In a particular embodiment,less moving parts are required which may lead to cost and weightreduction of the gas turbine engine 10. Furthermore, the moving parts ofsuch gearbox require lubrication. By having fewer parts, less oil may berequired. This may reduce the capacity of the required oil system and,because less heat is generated, the size of the required heat exchangerused to cool down the oil of the reduction gearbox 20 may be reduced. Ina particular embodiment, a total length of the gas turbine engine 10 maybe reduced by having the planetary gearbox 30 as described hereininstead of at least two conventional gearboxes disposed in series toachieve a speed reduction ratio equivalent to the one of the planetarygearbox 30.

In the illustrated embodiment, the turbine shaft 22 is connected to thesun gear 32. The propeller shaft 24 is connected to the connector 44 ofthe planet carrier 40, for instance by spline connection. In such anembodiment, corresponding to a planetary arrangement, rotation of thering gears 54 is limited as the ring gears 54 are fixed to a structureof the gas turbine engine 10 as shown in FIG. 2. It is understood thatlimiting rotation of the ring gears 54 comprises completely blocking therotation of said ring gears. The speed reduction ratio is defined as therotational speed of the shaft 22 over the rotational speed of the shaft24. Such an embodiment provides the highest speed reduction ratio andthe highest torque increase between the shafts 22 and 24 that ispossible to achieve with the planetary gearbox 20. In this arrangement,the shafts 22 and 24 rotate in the same direction relative to oneanother.

In an alternate embodiment, a star arrangement may be used. In a stararrangement, rotation of the planet carrier 40 is limited and thepropeller shaft 24 is operatively connected to the ring gears 54. It isunderstood that limiting rotation of the planet carrier 40 comprisescompletely blocking the rotation of said carrier. In this alternateembodiment, the ring gears 54 are both mounted and linked to thepropeller shaft 24. The total speed reduction ratio of the stararrangement would be less than the speed reduction ratio of the fixedconfiguration of the ring gears 54 as described above. In this alternateembodiment, the propeller shaft 24 and the turbine shaft 22 rotate inopposite directions.

By having two ring gears 54 disposed on opposite sides of the main gears46 the load is symmetrically distributed relative to a plane P, to whichan axis of rotation A of the sun gear 32 is normal, the plane P beinglocated half way through a thickness T of the main gears 46. Bysymmetrically distributing the load, the planetary gearbox may beadapted to withstand higher torques and may be adapted to use plainbearings instead of heavier and more expensive rolling element bearings.

The planetary gearbox 30 may be used in a plurality of applications,other than gas turbine engines, in which a rotational speed ratiobetween two rotating components is required. In such an embodiment, aninput is provided to one of the sun gear 32, the planet carrier 40, andthe ring gears 54 and an output is connected to another one of the sungear 32, the planet carrier 40, and the ring gears 54. Rotation of aremaining one of the sun gear 32, the planet carrier 40, and the ringgears 54, that is not connected to the input or the output, is limited.

The planetary gearbox 30 is adapted to change a rotational speed of arotating component relative to another rotating component. In theillustrated embodiment, the rotating component is the low-pressure shaft22 and the other rotating component is the shaft 24. In the illustratedembodiment, the shaft 24 is connected to the load 12, but it may beconnected to any other suitable component such as, but not limited to, ahelicopter rotor, or an accessory of the gas turbine engine 10.

To change the rotational speed of the shaft 24 relative to the shaft 22,the planetary gearbox 30 first receives a torque of the low-pressureshaft 22 via the sun gear 32. Then, the torque is transmitted to maingears 46 of a set of planet gear assemblies 36 meshed with the sun gear32. Each planet gear assembly 36 of the set of planet gear assemblies 36comprises aft and fore lateral gears 48 disposed on opposite sides ofthe main gear 46. In the illustrated embodiment, a first rotationalspeed ratio is generated by having a diameter 50 of the sun gear 32inferior to a diameter 52 of the main gears 46.

The torque is then transmitted from the fore and aft lateral gears 48 toone of the planet carrier 40 and the ring gears 54 meshed with the foreand aft lateral gears 48, while another one of the planet carrier 40 andthe ring gears 54 is fixed so as not to rotate. A second rotationalspeed ratio is generated by having the diameter 56 of the fore and aftlateral gears 48 inferior to the diameter 52 of the main gear 46. Thediameters 50, 52, and 56 may be tuned to achieve the desired reductionratio.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Still other modifications which fall within the scope of the presentinvention will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A planetary gearbox, comprising: a sun gear; a plurality of planetgear assemblies, each planet gear assembly having a main gear meshedwith the sun gear, a fore lateral gear and an aft lateral gear disposedon opposite sides of the main gear and rotating therewith, a diameter ofthe main gear being different than a diameter of the fore and aftlateral gears; a planet carrier rotatably supporting at least some ofthe planet gear assemblies; and at least one fore ring gear meshed withthe fore lateral gears, at least one aft ring gear meshed with the aftlateral gears, wherein one of the sun gear, the planet carrier, and thering gears is configured to be operatively connected to an input, one isconfigured to be operatively connected to an output, and rotation of aremaining one is limited.
 2. The planetary gearbox according to claim 1,wherein the diameter of the main gear is greater than the diameter ofthe fore and aft lateral gears.
 3. The planetary gearbox according toclaim 1, wherein a sun gear diameter is less than the diameter of themain gears of the planet gear assemblies.
 4. The planetary gearboxaccording to claim 1, wherein the main gears and the sun gear are spurgears.
 5. The planetary gearbox according to claim 1, wherein the ringgears and the aft and fore lateral gears are helical gears.
 6. Theplanetary gearbox according to claim 1, wherein the planet gearsassemblies are supported by shafts of the planet carrier, and bearingsare disposed between the shafts and the planet gears assemblies.
 7. Agas turbine engine comprising a compressor, a combustor and a turbine,the turbine having a first section for driving the compressor and asecond section driving a load, the second section of the turbineoperatively connected to a sun gear of a planetary gearbox, the sun gearmeshed with main gears of a plurality of planet gear assembliespivotally mounted on a planet carrier, the planet gear assemblies eachhaving a fore lateral gear and an aft lateral gear of a diameterdifferent than a main gear diameter, the fore and aft lateral gearsdisposed on opposite sides of the main gears, at least one fore ringgear meshed with the fore lateral gears, at least one aft ring gearmeshed with the aft lateral gears, one of the ring gears and the planetcarrier driving the load by rotation, while rotation of another one ofthe ring gears and the planet carrier is limited.
 8. The gas turbineengine according to claim 7, wherein a sun gear diameter is less thanthe main gear diameter.
 9. The gas turbine engine according to claim 7,wherein the main gear diameter is greater than the diameter of the foreand aft lateral gears.
 10. The gas turbine engine according to claim 7,wherein rotation of the ring gears is limited and the load isoperatively connected to the planet carrier.
 11. The gas turbine engineaccording to claim 7, wherein the load is a propeller.
 12. The gasturbine engine according to claim 7, wherein the first section of theturbine is in driving engagement with a high-pressure section of thecompressor and wherein the second section of the turbine is in drivingengagement with a low-pressure section of the compressor.
 13. The gasturbine engine according to claim 12, wherein the first section of theturbine and the high-pressure section of the compressor are mounted on ahigh-pressure shaft, the second section of the turbine and thelow-pressure section of the compressor are mounted on a low-pressureshaft, the low-pressure shaft in driving engagement with the sun gear.14. A method for changing a rotational speed of a first rotatingcomponent relative to a second rotating component, comprising: receivinga torque from the first rotating component using a sun gear;transmitting at a first rotational speed ratio a rotation of the sungear to a plurality of main gears of a plurality of planet gearassemblies rotatably mounted onto a planet carrier; transmitting atsecond rotational speed ratio a rotation of the main gears to one of theplanet carrier and ring gears while limiting rotation of another one ofthe planet carrier and the ring gears, the ring gears meshed to gearsdisposed on opposite sides of and rotating with the main gears; anddividing between the ring gears a load transmitted from the secondrotating component.
 15. The method according to claim 14, wherein eachof the main gears has a diameter greater than a sun gear diameter, themethod further comprising decreasing a rotational speed of the maingears relative to the sun gear.
 16. The method according to claim 14,wherein a diameter of the gears disposed on the opposite sides of themain gears is less than a diameter of the main gears, the method furthercomprising decreasing a rotational speed of the one of the ring gearsand the planet carrier relative to the gears disposed on the oppositesides of the main gears.
 17. The method of claim 14, wherein limitingrotation comprises limiting a rotation of the ring gears.